The following record relates to fleas collected from rats in and about Cambridge during the period 3rd March, 1909, to 20th November, 1911. We had difficulty in obtaining rats in Cambridge during the year 1911 owing largely to the high price asked by professional rat-catchers who usually sell them for sporting purposes. We have, consequently, included data collected by us during the two years before this investigation was undertaken on behalf of the Local Government Board.